In the original Shazam!, Superman showed up at the end of the movie to help Billy Batson aka Shazam’s brother out.
The movie will have no shortage of superheroes since the original Shazam! ended with Billy helping all his siblings gain superpowers to become the Marvel family.
Though I can confirm with ~90% certainty that Shazam will appear in Shazam 2.
Even though Superman might not show up in the Shazam! sequel, there is plenty for fans to get hyped about with regards to the film.